In animals with brains, the nervous system also generates and conducts thoughts and emotions. Thus it is the system that animates "animals" (sponges are an exception). Chemicals that target the activity of nerves generally are the most rapidly acting toxins, typically causing paralysis and/or death.
The nervous system consists basically of two types of cells:
Rapid signalling within the nervous system occurs by two primary mechanisms:
The nervous systems of vertebrate animals are often divided into a central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord. The PNS consists of all other nerves and neurons that do not lie within the CNS. The large majority of what are commonly called nerves (which are actually axonal processes of nerve cells) are considered to be PNS. The peripheral nervous system can also be roughly seen as consisting of sensory pathways and motor pathways , the latter being divided into the somatic (voluntary) nervous system and the autonomic nervous system.
The somatic nervous system is the voluntary part of the nervous system that coordinates a body's movements, such as maintaining a particular posture and walking. The autonomic nervous system is the involuntary part of the nervous system where all of the internal maintenance is taken care of.
The autonomic nervous system is then divided into the sympathetic division and parasympathetic division. The sympathetic nervous system responds to impending danger or stress, and is responsible for the increase of one's heartbeat and blood pressure, among other physiological changes, along with the sense of excitement he feels. The parasympathetic nervous system, on the other hand, is evident when a person is resting and feels relaxed, and is responsible for such things as the constriction of the pupil, the slowing of the heart, the dilation of the blood vessels, and the stimulation of the digestive and geniturinary systems.
The nervous system varies greatly among living animals. Cnidarians, such as sea anemones and jellyfish, have a nerve net, in which the activation of any one neuron triggers a wave of activation across the entire network. These waves are both afferent and efferent, representing both the sensation of chemicals or touch as well as the stimulus of the animals' reflexive mechanical response.The worms and flukes of the phylum platyhelminthes possess a network of nerves that operates like a more conventional nervous system, but lack a brain. Annelid worms and tunicates have a primitive brain called a ganglionic mass, which is a bundle of several nerves. Based in part on commonalities in embryonic development among all chordates, the tunicate brain is believed to resemble the evolutionary precursor of the vertebrate brainstem.